1. Field of the Invention
The present invention relates to a magnetic memory and a magnetic random access memory that employ spin torque magnetization reversal.
2. Description of the Related Art
In recent years, attention has been paid to a magnetic random access memory (MRAM) which can possibly replace the conventional dynamic random access memory (DRAM). As described in, for example, the specification of U.S. Pat. No. 5,734,605, the conventional MRAM employs the following recording system. Specifically, in a tunnel magnetoresistance effect element (TMR) having a multi-layer structure of magnetic film/non-magnetic insulating film/magnetic film, a magnetization of one of the magnetic films is reversed by using a synthetic magnetic field produced by current flowing into two metal wires formed in a direction perpendicular to the vertical direction of the TMR element. However, even in the MRAM, when the size of the TMR element is reduced to increase capacity, the size of the magnetic field necessary for magnetization reversal is increased, thereby requiring a large amount of current flowing into the metal wires. Thus, problems have been pointed out, including an increase in power consumption, and by extension, breakage of the wires.
A method for reversing magnetization without using the magnetic field is described in, for example, Journal of Magnetism and Magnetic Materials, 159, L1-7 (1996). It has been logically shown that the magnetization can be reversed only by making current of a predetermined value or more to flow into a giant magnetoresistance effect (GMR) film or a tunnel magnetoresistance effect (TMR) film, which is used in magnetic reproducing heads. After that, Physical Review Letters, Vol. 84, No. 14, pp. 3149-3152 (2000), for example, has reported an experimental example of the following recoding system. Specifically, a pillar having a multilayer (GMR film) of Co/Cu/Co and a diameter of 130 nm is formed between two Cu electrodes. Current is made to flow into the pillar. Spin of the flowing current gives spin torque to the magnetization of the Co layer, thereby reversing the magnetization of the Co layer. Moreover, in recent years, spin torque magnetization reversal has been demonstrated by use of a nano-pillar employing a TMR film, as described in, for example, Applied Physics Letters, Vol. 84, pp. 3118-3120 (2004). Being able to obtain output equal to or more than that obtained by the conventional MRAM, the spin torque magnetization reversal using a TMR film is particularly drawing much attention.
FIGS. 1A and 1B illustrate schematic views of the aforementioned spin torque magnetization reversal. In FIGS. 1A and B, a magnetoresistance effect element and one terminal of a transistor 6 are connected to a bit line 1. The other terminal of the transistor 6 is connected to a source line 7. The magnetoresistance effect element employed here includes a first ferromagnetic layer (free layer) 2 whose magnetization direction changes, an intermediate layer 3, and a second ferromagnetic layer (fixed layer) 4 whose magnetization direction is fixed. The conduction of the transistor 6 is controlled by a gate electrode 5. As illustrated in FIG. 1A, a current 8 is made to flow from the bit line 1 to the source line 7 in order to change magnetizations of the fixed layer 4 and the free layer 2 from an antiparallel (high resistance) state to a parallel (low resistance) state. At this time, an electron 9 flows from the source line 7 to the bit line 1. On the other hand, as illustrated in FIG. 1B, the current 8 is made to flow from the source line 7 to the bit line 1 in order to change the magnetizations of the fixed layer 4 and the free layer 2 from the parallel (low resistance) state to the antiparallel (high resistance) state. At this time, the electron 9 flows from the bit line 1 to the source line 7.
Further, for example, Nature, Vol. 425, pp. 380-383 (2003) has recently proposed the following spin torque oscillator including a magnetoresistance effect element which is formed by laminating a free layer 2, an intermediate layer 3 and a fixed layer 4, as illustrated in FIG. 2. Specifically, in order to excite precession in magnetization of the free layer 2, current Idc smaller than current necessary for spin torque magnetization reversal of the free layer 2 is made to flow into both sides of the magnetoresistance effect element. As a result, an alternating-current voltage of the element on both sides is excited through a giant magnetoresistance effect or tunnel magnetoresistance effect. Furthermore, for example, Nature, Vol. 438, pp. 339-342 (2005) has proposed the following new element. Specifically, as illustrated in FIG. 3, an alternating current with a frequency of the order of GHz is made to flow into a magnetoresistance effect element through a bias T31 to conversely take out a DC voltage Vdc.